Perfluorocarbon ionic exchange membranes provide high cation transport, and have been extensively used as ionic exchange membranes. Polymeric ion exchange membranes can be referred to as solid polymer electrolytes or polymer exchange membranes (PEM). Because of the severe requirements for fuel cell applications, the most commonly used membranes, and commercially available, are made from perfluorosulfonated Nafion®, Flemion® and Aciplex® polymers. However, reports and literature describe these membranes as working well but show several limitations that prevent developing the technology further to commercialization. Additionally, they work better with gaseous fuels than with liquid fuels which may be mainly due to liquid fuel crossover that diminishes cell performance. A membrane's chemical resistance and mechanical strength are important properties for fuel cell applications. Indeed, the membrane is often subjected to high differential pressure, hydration-dehydration cycles, as well as other stressful conditions. Also, mechanical strength becomes important when the membrane is very thin such as less than 50 microns. Further, when used with fuel cells or battery applications, the membrane sits in a very acidic medium at temperatures that can reach 200° C., in an oxidizing and/or reducing environment due to the presence of metal ions and sometimes the presence of solvents. This environment requires that the membrane be chemically and electrochemically resistant, as well as thermally stable.
Currently, many fluorine-containing membranes can suffer from one or more of the following short comings:
i) high liquid and gas crossover through the membrane;
ii) heterogeneous blending between the fluorinated polymer and other polymers that leads to inferior properties;
iii) insufficient chemical resistance in the presence of some liquid fuels;
iv) poor electrochemical resistance;
v) lack of homogeneous distribution of sulfonated groups;
vi) poor mechanical properties; and/or poor thermal stability.
Polyelectrolyte polymer blends having small domain sizes, and a process for producing such are described in US 2005077233. The polyelectrolyte polymer is a non-perfluorinated polymeric resin containing ionic and/or ionizable groups and in particular sulfonate or phosphonate groups, with a fluoropolymer matrix. One problem with the disclosed polyelectrolytes is that those containing hydrolytically unstable groups, such as esters and acrylamides, tend to hydrolyze in harsh chemical environments leading to a loss of the ionizable functionality.
WO 99/67304 describes a new class of unsaturated compounds having a fluoroether-substituted aromatic ring, and polymers formed from these compounds. One use for the polymers is as separators in electrochemical cells.
There is a need for a membrane that overcomes the limitations for use in fuel cell applications.
Surprisingly, it was found that polymer blends containing a fluoropolymer and a polyelectrolyte having no hydrolyzable groups can be used to form membranes for electrochemical cells having a high level of chemical resistance and mechanical strength.